This invention generally concerns a coupling for conducting both rotational movement and electrical current without the use of slip rings, and is specifically concerned with a small-diameter, low torque coupling that conducts rotational movement across a plurality of electrical wires while applying a minimum amount of stress on the wires.
Rotary couplings for conducting rotational movement across an array of electrical wires without the use of slip rings are known in the prior art. Such couplings find particular use in space applications, where telecommunication or electrical power currents must be transmitted over moveable joints on satellites, such as the joints in the support arms which connect a panel of solar cells to the main body of the satellite. Because the orientation of such solar panels must be constantly adjusted to maintain the cells in a perpendicular orientation with respect to the rays of the sun, the rotatable joints in such support arms must be capable of repeatedly rotating the panels 180 degrees to and fro while transmitting the electric power generated therefrom into the satellite body in a completely reliable manner. Slip rings have generally proved to be unsuitable for such power transmission as the high currents conducted through such slip rings tend to generate undesirable amounts of electromagnetic interference when the slip rings are rotated. Consequently, wire-conducting rotary joints have been constructed with the hope of achieving a rotary coupling capable of at least 180 degree movement in both a clockwise and a counterclockwise direction without the application of bending stresses on the wires passing therethrough which would cause these wires to either break or to short circuit as a result of repeated flexure.
One such wire conducting coupling is disclosed in U.S. Pat. No. 4,542,858. This design comprises a pair of relatively movable ring-type structures having plus or minus 180 degree relative rotation. The ring structures are interconnected by, inter alia, a pair of concentrically coiled metal bands which resemble intersecting watch springs. Electrical wires traversing the two ring-type structures are disposed between the two inter-nesting coiled metal bands. Sufficient slack is allowed in the portion of the wires captured between the two coiled metal bands so that little stress is applied to these wires when the coiled metal bands are wound tighter or looser due to rotational movement. In another design known in the art as a "twist-flex" coupling, two disc-like members are rotatably interconnected by means of an axially disposed shaft. A plurality of electrically conducted wires axially disposed with respect to the shaft are connected around the circumference of each of the circular members. Sufficient slack is incorporated into each of the wire segments disposed between the two circular members so that the outline of the structure generally resembles an hourglass. The wires generally remain spaced apart and parallel with respect to one another even when the two circular members are twisted with respect to one another, the only difference being that when some of the slack is pulled out of each of the wires due to rotation, the wires go from a parallel to an oblique orientation with respect to the longitudinal axis of the shaft of the coupling.
The demands made upon such wire-conductive, rotary couplings can be considerable. For example, in the orbiting Industrial Space Facility in the planning stages at the Westinghouse Electric Corporation, such couplings must be able to handle 200 or more 16 gauge wires for over 65,000 cycles between limits of plus and minus 180 degrees over ten years without breakage due to flexing. "Industrial Space Facility" is a trademark of Space Industries, Inc. Moreover, such coupling should take up a minimum of space and weight and should be able to perform its task without requiring any undue lengthening of the wire segments which traverse it that would result in unwanted electrical resistance. Finally, such a coupling should be rotatable upon the application of a minimum and uniform amount of torque. Unfortunately, neither of the aforementioned prior art designs completely fulfills these criteria. Rotary couplings utilizing concentrically coiled metal bands as previously described are not well suited to handle a large number of thick wire strands. While it is conceivable that the prior art designs of such couplings might be modified to handle greater numbers of electrical wires, such modifications would increase both the size and weight of the unit as a whole, thereby defeating one of the primary design objectives of the coupling. Additionally, the confinement of the wires between the metal bands impedes the ability of the coupling to safely dissipate the heat generated by the wires by virtue of electrical resistance. While the "twist flex" prior art design does have the ability to handle the number and type of wires necessary to conduct the current generated by a large panel of solar cells, the slack wire in the wire segments has a great deal of freedom of movement in the particular coupling which can lead to erratic wire movements and orientations that not only can increase or render nonuniform the amount of torque needed to twist the coupling, but can further increase the likelihood of wire breakage due to flexure. While ribbon-type wire conductors could be used which would minimize the problem associated with erratic wire movements, such ribbon conductors impede the repair of a broken wire within the coupling. Finally, the separate exterior micrometeorite shield that such prior art "twist flex" designs require increases both the diameter and overall weight of the coupling.
Clearly, there is a need for a wire conducting rotary coupling which is free of the shortcomings of the prior art, and capable of conducting a large number of heavy-gauge wires in a relatively stress-free manner in a structure which is both compact and lightweight. Ideally, such a coupling should require only very short amounts of slack wire to perform its functions so as to minimize power losses. It would be desirable if the wire segments in such a coupling were arranged so as to maximize radiative heat losses. Finally, such a coupling should be configured so as to provide a maximum amount of protection against micrometeorites without the need for a weight and diameter increasing housing, and include a means for closely controlling the movement of the slack portions of the wire segments so as to maintain the uniformity of the torque needed to turn the coupling and to minimize the probability of erratic wire movements which could stress and break the wires.